The production of cast dental restorations by the lost wax method is well known. This method generally involves the production of a wax pattern configured to match the desired restoration. This pattern is attached to a support commonly known as a sprue and the sprue is, in turn, secured to a base member.
The sprue, base member and wax pattern are placed within a container which is therafter filled with investment. After the investment has set, it is heated in order to melt the sprue and wax pattern which flow out of the investment through the opening created by the sprue and base, thereby leaving a passageway and cavity within the investment.
Molten alloys are then directed through the passageway created by the sprue and into the casting cavity created by the wax pattern. As the alloy solidifies, a casting is produced which is a duplicate of the original wax pattern.
In order to produce a a good casting, a variety of sprue assemblies have been developed. These various assemblies generally seek to create a smooth passageway so that the alloy may flow therethrough without turbulence which would cause porosity in the finished casting. In order to obtain a smooth passageway, plastic sprues, which burn out, were utilized. However, it was soon observed that when melting the plastic sprues, they would curl in such a manner that the investment wall along the passageway would be chipped or otherwise marked. In order to overcome this, hollow sprue pins wee developed which provided sufficient area for the melting plastic to curl inwardly, thereby avoiding the creation of excess pressure and subsequent marring of the passageway walls.
Another problem encountered relates to the shrinkage of the alloy as it cools. When the molten alloy filling the casting cavity cools, shrinkage occurs. In order to avoid irregularities in the final casting, there needs to be a source of molten alloy which can be drawn into the pattern cavity as the contents cool. To provide this source of molten alloy, sprue pins were formed with a bulbous area at a point toward the tip so as to create a reservoir of molten alloy within the investment. This alloy reservoir, being larger in size would solidify after the alloy within the cavity, thereby serving as a source of additional molten alloy to compensate for the shrinkage within the cavity.
It has been found, however, that even though the reservoir may contain molten alloy sufficient to compensate for shrinkage within the cavity, blockages often form which prevent alloy flow from the reservoir. Also, with the use of different alloys, the flow rates and solidifying times often vary from those established for high gold alloys. Thus, irregularities may occur with a new alloy that would not have occurred if high gold alloys were used.
During the development of the subject invention, it was therefore an object to provide a sprue pin which would allow for the flow of additional molten alloy to the pattern cavity during cooling.
Another object of this invention was to provide a sprue pin suitable for alloys which have different fluidity and different solidifying times.
A still further object of this invention was to provide a sprue pin which is sufficiently versatile to be effective with a variety of pattern sizes.
A still further object of this invention was to provide a sprue pin having a cavity and passageway structure that allows for gradual cooling in a predetermined sequence.
Other objects and advantages may be observed as the detailed description and drawings are considered as well as through practice with the invention.